Ethylene polymer film-type thermoplastic casings are known in many forms. For example, the casing may be a monolayer film of ethylene vinyl acetate (EVA), or very low density polyethylene (VLDPE or ULDPE) or blends thereof, suitable for packaging poultry. Alternatively, the ethylene polymer may comprise the outer layer of a multilayer film wherein at least one additional oxygen barrier-type polymer layer is adhered to the inside surface of ethylene polymer outer layer. These oxygen barriers may, for example, be vinylidene chloride copolymers of the saran type, or ethylene vinyl alcohol, or nylon. Most commonly, at least one additional layer is adhered to the inside surface of the oxygen barrier type polymer for direct contact with the encased product, which may be, for example, a food such as meat. Frequently this innermost third layer is gas-tight sealable to itself as for example by heat sealing or mechanical clipping. The innermost layer is often an ethylene polymer film and in some instances identical to the outer layer.
For most efficient storage, handling and stuffing of such ethylene polymer film-type casing, these casings are manufactured in the shirred form, i.e., compressed and pleated. When the shirred casing is to be filled with food it is placed on a stuffing machine where it is deshirred, filled with food and closed at opposite ends for example by clipping. The food package may then be processed at elevated temperatures usually in a steam atmosphere to cook the encased meat, for example frankfurter emulsion. Alternatively, the food package such as ham may be immersed in a hot water bath for processing. Often the casing is removed from the processed food body by high speed peeling equipment.
It will be apparent from the foregoing that the ethylene polymer film type casing is subjected to frequent distortions and resultant stresses during several handling steps, i.e., shirring, stuffing and in some instances peeling. The casing also experiences considerable friction during these operations. For example, during stuffing the deshirring casing may be pulled through an external holdback device which continuously provides frictional contact.
Typical external holdback devices, stuffing methods and apparatus are disclosed e.g. in U.S. Pat. Nos. 3,748,690; 4,164,057; 4,649,602; and 4,766,713 which patents are hereby incorporated by reference in their entireties.
If the casing film is the multilayer type there may be a tendency for the film layers to delaminate, particularly if they are held together only by innerlayer adhesion developed during extrusion. For example, a multilayer film may be formed by simultaneously melting resin in separate flow paths and flowing them as parallel molten streams to the extruder discharge end for interlayer adhesion. Alternatively, the multilayer film may be formed by coating lamination wherein additional layers are consecutively formed by melting and deposition on a thermoplastic substrate layer. If during any of the shirring, deshirring, stuffing or peeling steps there is a tendency for the multilayer film to delaminate, these high speed operations may exacerbate such tendencies causing areas of delamination. Delaminated film is an esthetic defect and highly undesirable to customers. Also, it is believed there is a greater tendency toward breakage in areas of delamination, particularly where delamination occurs at creases in the casing caused by prior reeling of the casing in a flattened condition or by shirring. Furthermore, during peeling operations it is believed that delamination exacerbates the problem of detecting unpeeled portions or fragments of casing which may be undesirably and unintentionally left on the peeled foodstuff. Thus, the outer layer or layers of a casing may be removed during peeling, but a segment of delaminated inner layer adjacent a foodstuff surface such as a meat surface may not be peeled off or may not even be slit by the peeler.
In addition to these concerns about multilayer film delamination, there is a danger of film tearing or breakage due to the aforementioned manipulative stresses and friction. Torn or broken film is generally unsuitable for stuffing with foodstuffs such as meat emulsion or cheese and breakage during a stuffing operation leads to processing down time, waste and loss of production.
For these reasons it has been common practice to provide an external lubricant coating on the food casing surface for the shirring, deshirring and stuffing steps. Mineral oil is most frequently used and most commonly applied as an external spray or by brushing during the shirring operation e.g. by spraying or brushing the shirring rolls with lubricant prior to their contact with the casing whereupon the coating transfers to the casing surface. Most commonly these casings have been formed of cellulose, both nonreinforced an fiber paper reinforced. When used in this manner, mineral oil has been quite effective on cellulosic casing as a semipermanent lubricating film which would remain intact on cellulosic shirred sticks during long storage periods. As such, the mineral oil was available as an external film to perform its lubricating function when needed during the shirring, deshirring and stuffing operations.
Surprisingly, it has been discovered that an external coating of mineral oil on ethylene polymer type film casings is not entirely satisfactory even though commercially used. More particularly, there have been instances where multilayer film-type shirred casing sticks with an ethylene polymer outer layer have been partially delaminated when removed from shipping/storage cartons by the food processor for positioning in stuffing systems. This was unexpected because there was no evidence of delamination when packaged in the carton by the casing manufacturer.
There have been other reported instances where stuffed but uncooked meat pieces had small delamination blisters, primarily located near the end closure clips. Delamination, again near the clips, has been observed after cooking but before peeling, with mineral oil externally coated ethylene polymer external layers. Delamination during the peeling operation has also been observed.
It has also been observed with mineral oil-lubricated ethylene polymer type film casings that the latter do not always move smoothly through stuffing machines as for example the SHIRMATIC type manufactured and sold by Viskase Corporation.
One object of this invention is to provide a shirred thermoplastic ethylene film casing article affording improved shirring, stuffing and peeling performance.
Another object of the present invention is to provide an improved shirred multilayer thermoplastic casing article formed of ethylene polymer film which casing has improved resistance to delamination.
A further object is to provide an improved method of manufacturing a shirred thermoplastic casing formed of an ethylene polymer tube.
Still another object is to provide an improved method for manufacturing a processed food body employing such a shirred thermoplastic casing.